As we have increasingly become an information society in recent years, optical disks, optical cards, optical tapes, and other such optical recording media have been proposed and developed as media that allow large volumes of information to be recorded and reproduced. Optical disks are of particular note as large-volume recording media. For instance, one type of optical disk is a phase change disk.
Recording to a phase change disk is performed by the following method. The recording layer is melted by being irradiated with a laser (the power here is called recording power) that raises the temperature of the recording layer over its melting point. Once the laser passes through, the melted region is quenched into an amorphous state, forming a mark. When the recording layer is irradiated with a laser (the power here is called erasure power) that brings the recording layer to over its crystallization temperature but under its melting point, the recording layer enters a crystalline state, forming a space. A recording pattern is formed, that is, recording is performed, on the phase change disk by continuing to form marks and spaces in this manner.
Meanwhile, reproduction from a recorded phase change disk is accomplished by irradiating the disk with a laser at a temperature below the crystallization temperature, and using a quad detector to detect the difference in the amounts of reflected light from the marks and spaces. Jitter is used here as a characteristic for expressing the quality of the reproduction signal detected from the phase change disk. The jitter of a reproduction signal from the grooves and lands of a phase change disk in land/groove recording is affected by the material and configuration of the disk and by the optical state of the disk as a whole, such as its initialization conditions.
Usually, with a phase change disk used for land/groove recording, the track pitch (this refers to the pitch between the grooves or between the lands) is constant within a given plane. Therefore, the optical state of the grooves and lands of a phase change disk can be expressed as the ratio (RG/RL) of the amount of light reflected from the grooves (RG) to the amount of light reflected from the lands (RL) when a medium in an unrecorded state is irradiated with a laser (hereinafter referred to as the reflected light ratio RG/RL). Because heat tends to build up in the grooves during recording to a phase change disk for land/groove recording, the value of the reflected light ratio RG/RL is set to be slightly greater than 1. Meanwhile, with a phase change disk with which recording is only performed in the grooves, the grooves are formed wider than the lands. There are also instances in which the land width of the lead-in and lead-out areas provided around the outer circumference of the user area is greater than the land width in the user area (see, for example, Japanese Laid-Open Patent Application H10-172183).
A phase change recording medium used only for low-speed recording has been marketed in the past as a phase change disk for land/groove recording. Furthermore, in order to record in less time as disk density has increased, phase change disks that can accommodate recording from conventional linear speeds up to 1.5 times these speeds have been developed and put on the market. All of these disks provide favorable jitter in which the reflected light ratio RG/RL value is about 1.05. More recently, phase change disks that can handle recording at a higher linear speed and at a plurality of linear speeds have also been developed.